1. Field of the Invention
The present invention relates to a method and device for continuous, electrical monitoring of the electrodes of an electrical heart stimulator, the heart stimulator including at least one stimulation electrode, one indifferent electrode, control electronics and an output stage for delivery of stimulation pulses to the stimulation electrode.
2. Description of the Prior Art
Damage to and other faults in the electrode system of an electrical heart stimulator can develop, making reliable heart stimulation impossible. During checks on implants at clinics and hospitals, the electrode system is also checked non-invasively. The magnitude of electrode impedance, i.e. stimulation impedance, and the appearance of pulses from surface ECG electrodes can supply information about electrode defects in the heart stimulator.
Such known methods, however, are uncertain and subjective and only reliable when "major" defects, such as electrode fracture, are present. The necessity that the check can only be made when the patient visits a clinic or hospital is also unsatisfactory.
A method and a device are described in European Application 0 437 104 for measuring electrode resistance with sub-threshold pulses supplied to defibrillation electrodes before a defibrillation shock is delivered. Only one current pulse with a predetermined magnitude is emitted across the electrodes and the resulting voltage is measured and the resistance calculated.
In U.S. Pat. No. 4,140,131 an implantable heart stimulator is described with two impedance detectors, i.e. a low impedance level detector and a high impedance level detector, which supply a warning if the stimulator's output impedance is outside a predetermined impedance range. The detectors compare the voltage measured across a series resistor in the electrode circuit upon the delivery of stimulation pulses with preset limit values.
Thus, the need for electrode monitoring of implantable heart stimulators is well-known and has hitherto been accomplished in different ways, such as direct measurement on the electrode, with no stimulator connected, or by delivery of stimulation pulses, the impedance of pulses being calculated with some appropriate measurement method. The measurement value is then transmitted by telemetry, and the doctor decides whether the impedance value designates a fault-free electrode system.